1. Field of the Invention
The present invention relates to an optical deflector deflecting and scanning light beams such as laser beams, and more particularly to an optical deflector using piezoelectric force. Further, the present invention relates to an optical scanner using the optical deflector, an image forming apparatus using the optical scanner as an optical writing unit, and an image projector using the optical deflector as a scanning unit.
2. Discussion of the Background
Optical deflectors deflecting and scanning light beams such as laser beams are widely used in image forming apparatuses such as copiers, image projectors, laser beam printers, and barcode scanners. As such optical deflectors, optical deflectors using an electrostatic force, an electromagnetic force, and a piezoelectric force are conventionally known.
The optical deflectors using an electrostatic force include those with an electrode having the shape of a parallel plate and with a pectinate electrode. The optical deflector with a pectinate electrode can generate a comparatively large drive force owing to recent improvement in fine processing technology. However, the optical deflector with a pectinate electrode cannot obtain a sufficient deflection angle of a light beam and has to make up for that with a large drive voltage. However, the parts needed for such an electric source become large to enlarge the drive voltage, resulting in growth in size of the deflector and an increase in cost.
As for the optical deflector using an electromagnetic force, the magnetic force of a permanent magnet or a current of the coil needs increasing, resulting in growth in size of the deflector and an increase in electric power consumption. A deflector using a magnetostrictive film is available to downsize the deflector, but has poor magnetic properties. When current flows through the coil, extra heat is generated, resulting in an increase in electric power consumption as well.
Meanwhile, although the deflector using a piezoelectric force needs a comparatively large drive voltage, it can generate a large force with a small amount of electric power. Thus, when the piezoelectric material is laminated to a beam-shaped elastic member to have a unimorph structure or a bimorph structure, a slight distortion in an inner surface direction due to the piezoelectric force is changed to a warpage to obtain a large deformation. However, the conventional optical deflectors using a piezoelectric force still have various problems.
Japanese Patent No. 3129219 discloses an optical deflector oscillating a whole oscillatably supported frame with a bulk piezoelectric element to rotationally oscillate a micro mirror. However, the bulk piezoelectric element applies an oscillation having a small amplitude and is difficult to make compact, resulting in an increase in cost.
Japanese Patent No. 3246106 discloses an optical deflector in which each end of a pair of piezoelectric bimorphs is fixed on a substrate in the shape of a cantilever beam, free ends thereof are connected with each other with a connection member, and a mirror is located on a torsional deformation member extended from the center of the connection member parallel to the piezoelectric bimorph. Having a torsional deformation member with a free end, the optical deflector can deform in a bending direction as well as a torsional deformation direction, and the mirror is rotatable in a biaxial direction. However, since the torsional deformation member is parallel to the piezoelectric bimorph, a moment generated by bending of the bimorphs is not fully used. As for movement in a bending direction perpendicular to the torsion, since the piezoelectric bimorph exhibits such a small deformation amount that it is difficult to deform in a bending direction when a frequency is increased to increase rigidity of the torsional member, it is difficult to obtain a large amplitude in the movement in a bending direction.
Japanese published unexamined patent application no. 2008-083603 discloses an optical deflector in which a pair of elastic support members connected with both sides of a mirror, oscillatably supporting the mirror, are supported by a pair of drive beams (cantilevers) formed of beam-shaped elastic members laminated with a piezoelectric material at a right angle to an axis of the elastic support members, and the pair of drive beams are driven in reverse phase to rotationally oscillate the mirror. However, since the mirror and the mirror support members are supported by the pair of drive beams, the mirror has a limited rotational amplitude. In addition, four drive beams are needed for rotational amplitude in one direction, resulting in difficulty in downsizing and an increase in cost.
Japanese published unexamined application no. 2004-252337 discloses an optical scanner in which each end of a pair of plate-shaped deformation members circularly curved is fixed on a fixed part, a mirror is supported by another end through a support member, and the pair of formation members are elastically deformed to deflect the mirror. When the structural body is formed of a soft material such as polyimide, a comparatively large amplitude angle can be obtained. However, a material such as silicon can obtain only a slight amplitude angle because a piezoelectric unimorph structure exhibits only a very small deformation amount upon voltage application to the piezoelectric material. The structures of FIGS. 17 and 18 therein accumulate deformation to obtain a large amplitude, but are too large to move quickly.
For these reasons, a need exists for a compact optical deflector having good drive efficiency and a large rotational amplitude.